Out of interest, if you run this using a cron job, what happens after the call has been placed - does the server keep running? I've had a need for a sort-of volatile web server to respond to requests like this before.
The server will keep running if you don't kill it, but it shouldn't matter too much because the port will be occupied and when it runs the next time, the new instance will error out, and Twilio will contact the old server (if it's still running).
Does that sound reasonable?